As a traffic of a communication network drastically increases, there has been a continuous demand for downsizing and power saving of an optical transceiver. A reception part of the optical transceiver includes a photodetector such as a photodiode (PD) which converts an optical signal into a current signal (photocurrent), and a transimpedance amplifier (TIA) which converts the current signal into a voltage signal and amplifies the voltage signal. The current signal generated by the PD is not only used to demodulate a signal modulated by a transmission part of the optical transceiver but also used to monitor an intensity of a received optical signal and detect a no signal (loss of signal) state.
An example of a configuration for monitoring the intensity of the optical signal is disclosed in Japanese Patent Application Laid-Open No. 11-40840 in which a current mirror circuit connected in series with a PD is used for a current monitoring circuit. Another current monitoring circuit is disclosed in Japanese Patent Application Laid-Open No. 2009-260300 in which an amount of a current signal is calculated from a voltage drop generated at a resistor connected in series with a PD.
The current mirror circuit mentioned above needs a PNP-type bipolar transistor, a P-channel FET (Field Effect Transistor), or the like. On the other hand, the transimpedance amplifier is configured to include an NPN-type bipolar transistor in order to secure high speed performance. Generally, since the PNP-type and NPN-type bipolar transistors are difficult to integrate on the same chip, each type of transistor should be manufactured on a separate chip. This is not preferable in terms of the downsizing of the transimpedance amplifier. Further, in the configuration including the current mirror circuit, a transistor needs some voltage between an emitter and a collector of the transistor, which is not preferable to reduce power consumption. Similarly, also in the configuration having the resistor connected in series with a PD, a large supply voltage is necessary to generate the voltage drop at the resistor. Such a large supply voltage is not preferable in terms of the power saving.